Since 2002, the Simon Wiesenthal Centre, based in Los Angeles, has run an international campaign to bring the last surviving Nazi war criminals to justice, while there is still time. But "Operation Last Chance" is in danger of backfiring and bringing the search for justice into disrepute. This is not because the moral arguments have weakened one iota. They are as powerful as ever. Rather, it comes down to the practical obstacles in the way of fair trials more than 60 years after the alleged offences were committed.

There can be no statute of limitations on the crime of murder, let alone mass murder. Several of those on the Centre's list were guards at concentration camps or took part in mass shootings. Milivoj Asner, 94, was a Croat police chief who deported Serbs and Roma, as well as Jews, to murderous camps. The suspects are very old, but they showed no mercy to their victims on the grounds of age. Aribert Heim, 93, who injected phenol into the hearts of camp inmates did not distinguish between youths and pensioners.

Time does not erase the guilt of past crimes. Indeed, were the passage of time and the age of the perpetrators used as a reason for non-prosecution it would signal the genocidaires at work today that all they need to do is keep their heads down for long enough and they will escape retribution. With a slow genocide underway in Darfur, and Kosovo on the boil again, this is not a desirable message. If we renege on justice for the victims of genocide, it suggests that their lives did not matter, condemning them to a double death. We would also forfeit the opportunity that the courtroom affords to give voice to the survivors and to gather the evidence of atrocities that will serve later historians and offer lessons for the future.

However, these arguments assume that prosecutions can be brought, that trials will be fair, and the verdicts sound. This is asking a lot. In England, Australia, Canada, and the US, where there have been judicial proceedings against suspected Nazi war criminals, it has taken years to build each case and gather sufficient credible, reliable, and admissible eyewitness evidence to ensure a good chance of conviction. Preliminary hearings, followed by full-scale trials, have stretched over several years. Given that Heim and Asner are well into their 90s, it is unlikely that a judge would allow proceedings against them to go ahead. There is the health and resilience of witnesses to consider. They may have to be interviewed several times, and travel long distances to testify. Yet none with credible memories can be less than 80 years old.

To initiate criminal proceedings under these circumstances would mean reliance on paper evidence or shaky eyewitnesses. In either case, the outcome could not be considered safe. If a conviction was secured, it would be hard to persuade a sceptical public that the result was not foreordained. This would invite a backlash with the opposite effect to that desired. Yet, the Simon Weisenthal Centre has already done much to ensure that Nazi war crimes are inscribed in popular consciousness around the world and changed attitudes towards the mass abuse of human rights. If this helps to ensure that, in future, justice is done against those who commit crimes against humanity, then past failings of the judicial system may be forgiven - even if the perpetrators are not.